1. Field of the Invention
The present invention relates to actuators, and more particularly to the drive for actuators, which may contain two or more inputs, one of which may be many times larger than the other.
2. Description of the Prior Art
Actuators which may be used, for example, to position a device in a desired attitude are well-known in the art. The device is positioned by a first signal, herein called the coarse signal, usually from a computer through an amplifier to the actuator. The device being positioned may also be subject to undesirable changes of position due to, for example, vibration. To avoid this problem, an additional actuator has been heretofore employed to provide a second positioning signal, herein called the fine signal, to counteract the vibration. The fine signals may be several orders of magnitude smaller than the coarse, and so the problem of noise in the coarse signal presents a special problem since it may be in the same general magnitude as the fine signals. If an attempt to drive the actuator from a combination of the coarse and fine signals is made, the noise in the coarse signal may swamp the fine signal so that the fine signals are lost. If the noise is filtered out by a low pass filter, then the fine signals may very well be filtered out also. Using a separate actuator for the fine signals is expensive, heavy and often nearly impossible without adding a second platform or stage for the device being positioned.